The invention is based on a dual-circuit brake system having a pressure sensor for monitoring the brake pressures occurring at various times in the two brake circuits, the pressure sensor being equipped with a double piston which is subjected to the pressure of the two brake pressures and with an electrical switch disposed between the two pistons of the double piston and actuated via oblique faces and a push rod. A brake system of this kind is known (U.S. Pat. No. 4,030,785). The Leiber U.S. Pat. No. 4,141,595 is also considered to be of interest relative to its electronic control unit.
In this known dual-circuit brake system, the pressure sensor provides that if one brake circuit fails, a switch will close an electrical circuit so that the driver will learn of the defect by way of an optical and/or acoustical signal. The known design provides no impetus to make a combination with other, preferably electronic, control units for the purpose of further evaluation of the switching signals.
A further factor is that modern anti-wheel-locking systems, also known as anti-skid braking systems, which are primarily designed using integrated circuitry, include a test circuit which monitors the electronic function and the electrical connection of the components, for instance at the beginning of a drive. According to the present state of the art, the hydraulic functions of such a system are not monitored. In principle, this would be possible by means of the installation of a specialized pressure sensor in each individual regulating conduit; however, because of the expense (pressure sensors and electrical lines leading to the control unit), this is not done. So-called "diagnosis contacts" have also been provided on a magnetic valve actuated by the magnetic armature. However, the expense for such a provision is again not insignificant. Furthermore, in the latter case only part of the control member, namely the armature movement, is monitored, which does not necessarily mean, however, that the hydraulic valve actuated by the armature is also functioning correctly.